Nanophosphor LaF3:Ce has been synthesized and incorporated into a matrix to form a nanocomposite
scintillator suitable for application to γ-ray detection. Owing to the small nanocrystallite size (sub-10 nm),
optical emission from the γ / nanophosphor interaction is only weakly Rayleigh scattered (optical attenuation
length exceeds 1 cm for 5-nm crystallites), thus yielding a transparent scintillator. The measured energy
resolution is ca. 16% for 137Cs γ rays, which may be improved by utilizing brighter nanophosphors. Synthesis of
the nanophosphor is achieved via a solution-precipitation method that is inexpensive, amenable to routine
processing, and readily scalable to large volumes. These results demonstrate nanocomposite scintillator proof-of-
principle and provide a framework for further research in this nascent field of scintillator research.
Nanophosphors correspond to nanostructured, inorganic, insulating solid materials that emit light under particle or
electromagnetic excitation. Although extensive investigation of the optical properties of nanostructured semiconductors
is underway, nanophosphors remain largely unexplored. Nanophosphor Tb-doped Y2O3 was obtained by the solution
combustion technique with Tb concentrations up to 5 at.%. Structural characterization, assessed by transmission electron
microscopy and x-ray diffraction, show the existence of nanoparticles with a cubic crystallographic structure and sizes in
the 30 to 70 nm range. As a result of the combustion process, the nanoparticles agglomerate into large micron-sized
entities. Photoluminescence emission and excitation spectra obtained at room temperature show distinct differences in
the optical behavior of the bulk and nanomaterial. Specifically, the excitation spectra of the nanophosphors are
systematically blue-shifted relative to bulk spectra. The photoluminescence emission spectra, which originates from
5D4→7FJ transitions comprising several sharp lines in the visible spectrum, also exhibit contrasting behavior upon Tb
incorporation; the 5D4→7F5,6 intensity ratio decreases with increasing Tb content in the bulk but is constant in the
nanophosphor. Finally, the maximum in the quenching curve of the nanostructured material occurs at 1.5 at %, which is
three times higher than for the bulk material.
Cerium-doped lutetium oxyorthosilicate (LSO:Ce) is an excellent scintillator due to its fast decay time (approximately 40 ns), excellent brightness (> 3X bismuth germanate), and high density (7.4 gm/cm3). However, the luminescence process in this technologically important scintillator is not well understood. Elucidation of electronic traps and their role in scintillator afterglow is lacking and fundamental ion-lattice coupling parameters have not been established. From thermally stimulated luminescence and emission spectra data on several oxyorthosilicates we show the traps to be intrinsic and uniquely associated with the C2/c crystal structure. Temperature dependent optical absorption measurements reveal prominent Gaussian bands at 3.432 eV (peak a), 3.502 eV (peak b), 4.236 eV (peak c) and 4.746 eV (peak d). The second moments are well described by the usual linear coupling model yielding the Huang-Rhys parameter and vibrational quantum energies for each peak. Oscillator strengths of the 4f yields 5d transitions are calculated from Smakula's formula and knowledge of the cerium distribution between the two crystallographically inequivalent sites. From the known correlation between average Ce-ion- ligand distance and oscillator strength we conclude that peak a is correlated with the seven-oxygen-coordinated site, and peaks b, c and d are associated with the six-oxygen- coordinated site.
D. Wayne Cooke, Bryan Bennett, Ross Muenchausen, J. Thomas, Nigel Cockroft, John Quagliano, Roger Petrin, C. Maggiore, M. Paffett, David Wayne, T. Taylor, M. Hawley, John Jacco, Michael Scripsick
The origin of optical damage in potassium titanyl phosphate (KTP) crystals has been vigorously investigated since its introduction as a nonlinear optical material in 1976. It is well known that this material exhibits a laser damage threshold that limits its use in many high average-power applications, especially frequency doubling of Nd-doped lasers. Both photochromic and electrochromic damage can be induced in KTP. Until recently, it was thought that these two types of damage were distinctly different, possibly involving different mechanisms; however, new data show that electrochromic-like damage can be induced in KTP by laser irradiation only, implying the existence of an internal electric field.We have recently observed bursts of light when heating KTP crystals at 0.1-1.0 K/s in the temperature range 8-675 K. The scintillations correspond to molecular nitrogen emission occurring during the electrical breakdown of air near the crystal surface, and imply the existence of pyroelectric fields in KTP exceeding 30 kV/cm. These fields wee induced by 10.6 micrometers laser irradiation. The observation of pyroelectric effects, heretofore not considered in KTP damage models, provides an important new insight into the possible cause of the recently observed 'electrochromic- like' photochromic damage in KTP.
High Tc superconducting thin films of YBa2Cu3O7-(delta ) (YBCO) were deposited on sapphire substrates with buffer layers of yttria-stabilized zirconia and cerium oxide by using pulsed laser deposition (PLD) technique. The epitaxial relationship between the substrate and buffer layers as well as the buffer layer and the superconducting film was established. Furthermore, using the same thin film technique, YBCO superconducting thin films were deposited on both sides of substrates of up to 5 cm in diameter. The superconducting properties of the films on both the sapphire with buffer layers and large-area substrates were comparable to the ones of the best YBCO films.
One and two inch diameter wafers of (100) LaAlO3 have been coated with
thin films of YBa2Cu3O7_ by a pulsed laser deposition technique.
Deposition parameters have been optimized to produce uniform, 90 K films
which have surface resistance values between 0.4 and 0.8 mμ at 4 K and
22 GHz.
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